Ophthalmoscopes are well known instruments used for eye examinations. A direct ophthalmoscope is a device which has an aperture which defines a line of sight and means for projecting a narrow beam of light in substantial alignment with the line of sight for illumination. The light is directed through the pupil of the eye so that the interior and, in particular the fundus of the eye may be examined.
Typically, a direct ophthalmoscopic procedure includes cyclopleging the eye to dilate the pupil for better viewing. The examiner then directs the projected light beam through the pupil and peers through the peephole into the examinee's eye. Generally, direct ophthalmoscopes include a plurality of lenses which may be rotated or otherwise positioned in the path of the peephole line of sight to focus and/or magnify the interior surface of the eye which is being examined. Some direct ophthalmoscopes provide means for varying the size and intensity of luminosity of the light to adjust the illumination of the interior of the eye to the examiner's preference.
One fundus structure which is typically examined is the optic disc, the structure formed where the optic nerve connects to the eye. Generally the optic disc includes an optic cup and the relative size of the optic disc and cup are easily observed. Conditions such as megalopapilla disc (a gross enlargement of the disc) and hypoplastic disc (an exaggerated diminished disc size) are readily determined through conventional direct ophthalmoscopic examination. Additionally, a skilled examiner can determine through observation the ratio in size between the optic disc and optic cup in order to obtain the relative size of the area of neuroretinal rim which is the area of the disc surrounding the cup.
Other procedures have been developed to obtain a relative measurement of the optic disc and other fundus structures through the use of a retinal graticule. This procedure entails the projection of a grid onto the fundus of the eye and counting the number of squares of the grid occupied by the fundus. The procedure is repeated in subsequent examinations to denote any relative change in the size of the optical disc based on the difference in number of grid sections observed in the first and subsequent examinations.
Grid projection has been accomplished by mounting a slide within the path of the light beam on an adjustable arm and positioning the slide to focus the grid on the fundus. Although research has been done in the calculation of the actual size of the grid squares projected, this procedure has not generally been used to determine a quantitative measurement of the diameter and/or area of the optic disc or other fundus structures. See Morgan, O. G., "A Retinal Graticule", British Journal of Ophthalmology, v.11, p. 339, 1927.
One problem in attempting to quantitatively measure fundus structures using grid projection is the fact that the fundus is not flat and the grids cannot be focused over the entire surface. Another apparent problem of quantitatively measuring fundus structures through direct ophthalmoscopic examination is the fact that the eye being examined is a lens and eyes generally vary in size and optic power.
Conventionally, where a quantitative measurement of the optic disc and, in particular, the area of neuroretinal rim is desired, stereoscopic instruments and procedures are used. Such methods entail the need for trained specialists and specialized equipment to achieve an accurate measurement of the desired fundus structure. One known method involves stereoscopic photography; another involves stereoscopic video recording of the fundus, such as with an Optic Nerve Head Analyzer by Rodenstock. Both methods are time consuming and expensive. Accordingly, it is desirable to provide an ophthalmoscopic procedure and ophthalmoscope to obtain a quantitative measurement of fundus structures.